1. Field of the Invention
The present invention relates to a surge arrester having a fail-safe structure for permanently shorting the arrester to ground after an extended discharge and in particular to such a device having an improved spark gap structure as a back up to the fail-safe device.
2. Description of the Prior Art
A spark gap protector utilizing a gas tube spark gap device and a spring actuated fail-safe device for permanently shorting the spark gap protector to ground after an extended discharge is known, for example, from U.S. Pat. No. 4,132,915 and the corresponding German OS No. 27 40 695. As disclosed therein, the gas tube spark gap device has a flanged ring connected to one of its two electrodes which forms the first electrode of an auxiliary spark gap structure, an insulating ring having two apertures therein which is seated on a face of the flanged ring, and an annular metal element disposed on the opposite side of the insulator ring which forms the second electrode of the auxiliary spark gap structure through the apertures in the insulator ring. The second electrode of the auxiliary spark gap structure is further connected via contact springs with the other electrode of the gas tube spark gap device. The teachings of U.S. Pat. No. 4,132,915 are incorporated herein by reference.
Within the gas tube spark gap device is a solder pellet which is melted upon an extended discharge of high voltage permitting a bias spring to urge the housing containing the gas tube spark gap device into a permanently shorted fail-safe condition. For proper operation thereof, the gas tube spark gap device must remain sealed, and in the event of a fracture to the gas tube structure the auxiliary spark gap structure provides a back up protection at a breakdown voltage which is larger than the breakdown voltage ordinarily associated with the gas tube spark gap device.
This structure of a gas discharge surge arrester in parallel connection with the auxiliary spark gap structure is particularly suited for the protection of telephone installations against surge voltages.
The above-described known combination of a gas discharge surge arrester with an auxiliary spark gap structure has the disadvantage that the air gap within the auxiliary spark gap structure is approximately 0.1 mm which is defined and limited by the thickness of the insulating ring. For reasons well known to those skilled in the art, discharges within the gap tend to occur at the edge of the apertures in the insulator ring, and after a number of such discharges a conductive coating is generated on the surface of the insulator ring at the aperture edges thereby reducing the insullation capabilities in that region. The load capacity and proper operation of the entire structure are thereby diminished due to the increased conductivity between the two electrodes of the spark gap structure.